1. Field of the Invention
The present invention relates to lasers and more particularly to tunable ytterbium-doped solid state lasers.
2. Description of the Related Art
It is desired to develop tunable lasers that operate in the wavelength ranges of about 1020 nm to about 1050 nm, and about 510 nm to about 525 nm.
Neodymium solid-state lasers (e.g. Nd:YAG) are widely used in a variety of applications. One problem with neodymium lasers is that they have moderate wallplug-to-output efficiency. Neodymium has a narrow absorption band (about 3 nm). Consequently, the pump diodes must be carefully engineered and cooled to keep them at the same wavelength. Precise temperature control is required, consuming a great deal of power for refrigeration. Moreover, neodymium has a short fluorescence lifetime, about 230 .mu.s. For a diode-pumped laser, this dramatically increases the cost of the system, since a large number of expensive diode arrays are required for operation.
Ytterbium solid-state lasers have advantages over Nd lasers. See U.S. Pat. No. 5,123,026, issued Jun. 16, 1992 to Fan et al., which is incorporated herein by reference. This patent discloses a frequency-doubled, diode-pumped ytterbium laser operating at 515 nm (frequency-doubled laser oscillation at 1030 nm). To confine oscillation to 1030 nm, Fan '026 teaches the use of a fixed wavelength discrimination element for suppressing laser oscillation at 1048 nm. As will be explained infra, this is a less preferred configuration for a ytterbium solid-state laser.
Tunable lasers are desired in the laser art. See U.S. Pat. No. 4,969,150, issued Nov. 6, 1990 to Esterowitz et al., which is incorporated herein by reference. This patent discloses a tunable cw thulium-doped solid-state laser.
Lasers for point-to-point communications are desired in the laser art. One desired feature of a laser for point-to-point communications is a high signal-to-noise ratio. Solar radiation is the major source of noise for above-ground point-to-point laser communications.
Examination of the solar spectrum during the 19th century revealed that the spectrum departs from a continuous spectrum as would be expected for a simple blackbody radiator. Fraunhoffer was the first to report that there were dark lines, i.e. narrow spectral regions where there was little apparent radiation compared to adjacent spectral regions. This phenomena was understood to be caused by elements in the solar corona, e.g. hydrogen, absorbing the radiation from the element in the core of the sun. The less than total absorption and the shape of these lines are caused by the differing regimes of pressure and temperature in the core and corona.
Lasers for underwater communications are desired in the laser art. Only for a range of wavelengths between about 450 nm and 540 nm is oceanic water transparent enough to allow reasonable signal levels to be achieved for even moderate depths for a submerged platform. A number of the Fraunhoffer lines are in this spectral range. Operation of a communications system at one or more of these wavelengths could take advantage of the improvement in signal to noise level offered by a noise level which can be 5-10 times lower than at other wavelengths.